The present invention relates to turbo compressor having a plurality of impellers for compressing a gas such as air and, more particularly, to a turbo compressor for compressing a gas at a high flow speed and a large compression ratio.
In general, a turbo compressor has a plurality of centrifugal impellers carried by an impeller shaft in a plurality of stages through which the gas such as air is progressively compressed and then discharged. Conventional multi-stage turbo compressors for compressing a gas with large compression ratio and at large flow rate are complicated in construction, and include a large number of stages, i.e., impellers, in order to avoid any reduction in the efficiency. In order to avoid this problem, U.S. Pat. No. 4,224,010 proposes a turbo compressor in which the angle formed between the axis of the impeller shaft and the direction in which the compressed gas is discharged from an impeller is less than that in the impeller immediately downstream from the impeller, when viewed in a meridional plane. This conventional construction provides a turbo compressor which operates with a comparatively high operating efficiency and has a comparatively small size, but cannot satisfactorily meet current demands of engineering firms and end users for turbo compressors which operate with high levels of compression ratio and flow rate. Namely, in order to handle a large amount of gas, it is necessary that impellers of high specific rate are arranged on a single impeller shaft in a multiplicity of stages. On the other hand, high pressure ratio requires that the peripheral velocities of the impellers are increased or, alternatively, the number of the impellers is increased. Unfortunately, however, there is a practical limit in the peripheral velocity of the impeller from the view point of the mechanical strength of the impeller material. This naturally leads to a conclusion that the number of the impellers be increased. A greater number of impellers requires a correspondingly greater length of the impeller shaft, posing problems or difficulty in regard to the critical speed and dynamic behavior of the rotor. For these reasons, known turbo compressors can not satisfactorily meet the demands for greater flow rate and higher pressure ratio.
A diagonal flow type compressor is mentioned in Proceedings of the Sixth Turbo machinery Symposium (October 1977), pp 61-62. A compressor also is known in which diffusers are bent in radial directions, i.e., perpendicularly to the axis of the impeller shaft, as reported in Bulletin of the Japan Association of Mechanical Engineering, March 1987, pp 16-20.
In general, when the specific speed ns, that is the ratio between the diameter of the inlet end of the impeller and the diameter of the outlet end of the impeller is increased so that curvature is increased when centrifugal impellers are used, the performance of the compressor is impaired. The specific speed ns is determined in accordance with the following relationship: EQU ns=N.multidot..sqroot.Q/H.sub.od 0.75,
where:
N=a rotational speed (rpm), PA0 Q=a volumetric flow rate of gas (m.sup.3 /min), and PA0 H.sub.od =an adiabatic head (m).
In the case of the diagonal flow type impeller, the curvature of the flow path in meridional plane is small so that a substantially uniform breadthwise flow distribution is obtained at the outlet of the impeller, i.e., at the inlet of the diffuser, thus eliminating offset of the flow of the gas towards a core plate. However, if the flow of the gas entering the diagonal flow diffuser has a rotational component, the flow of the gas is offset towards a side plate in the region between an intermediate portion and the outlet of the diffuser due to the curvature in the direction perpendicular to the direction of path of the gas. When this offset tendency is enhanced to an extreme level, a reverse flow may be produced around the core plate so that the diffuser loss is drastically increased. A diagonal flow type compressor employing a diffuser of the type described above inevitably has a large axial length so that the friction loss along the flow passage is increased correspondingly. In addition, the critical speed of the shaft system is undesirably lowered due to an increased length of the impeller shaft. In order to overcome these problems, a compressor has been proposed in which guide vanes of a height of 10 to 50% of the meridional plane are provided in the diffuser at positions adjacent to the core plate as disclosed, for example, in Japanese Utility Model Laid-Open Publication No. 56-38240. Such guide vanes, however, cannot provide satisfactory effect. Thus, problems such as difficulty in the reduction of size, increase in the friction loss, lowering of the critical speed and so forth still remain unsolved.